1. Technical Field
The present invention relates to an azimuth processing device, an azimuth processing method, an azimuth processing program, a direction finding device, a tilt offset correcting method, azimuth measuring method, a compass sensor unit, and a portable electronic device. In particular, the present invention relates to estimating and updating of an offset of the compass sensor.
2. Background Art
Some recent portable information terminals, such as cellular phones and PDAs, have a function for displaying geographic information using GPS and a compass sensor. For example, a portable information terminal is known, which determines its current position using GPS, downloads map information around the current position from a server via a communication line, determines azimuth heading with a built-in compass sensor, and displays the map information on a screen in such a manner to orient the map heading to the actual azimuth heading. The compass sensor detects a magnetic field of the Earth to measure an azimuth angle, but the fact is that it detects a mixture of the Earth's geomagnetic field and noise magnetic fields leaked from a loudspeaker, a microphone, a metallic package for electronic parts, etc. mounted in the portable information terminal. Therefore, in order to determine the azimuth angle accurately, it is necessary to determine the noise magnetic fields (offset) other than the magnetic field of the Earth and correct measurement data from the compass sensor by the determined offset.
Patent Document 1 discloses a method of estimating and updating the offset of a compass sensor. An offset updating method disclosed in this publication is to acquire measurement data output from the compass sensor at intervals of 90 or 180 degrees while a user rotates the compass sensor more than 90 or 180 degrees about a specific axis so as to calculate the offset of the compass sensor based on the acquired measurement data. To calculate the offset of the compass sensor accurately, the user has to, for example, rotate on a horizontal plane the equipment with the compass sensor in it, or heavily shake it up and down and left and right, so that measurement data upon which the calculation is made will be output from the compass sensor. The operation for allowing the compass sensor to output measurement data necessary to update the offset of the compass sensor is called calibration. It is ideal that the calibration is performed by changing the attitude, or position, of the equipment to a large degree at a constant angular speed. Hereafter, the term “calibration” specifically means the steps of calculating an offset based on measurement data acquired from a compass sensor, and updating a previous offset of the compass sensor by the calculated new offset data.
However, the user might drop the equipment during the calibration. Further, even when the user completed calibration, accurate offset might not be able to be calculated from measurement data from the compass sensor accumulated during the calibration. Furthermore, the calibration procedure is hard to perform correctly without reading the manual or the like. Since the procedure is necessary only to update the offset of the compass sensor, it is burdensome for the user to follow. Accurate geographic information based on accurate azimuth data cannot be displayed unless the offset of the compass sensor is updated correctly.
Further, in order to find an accurate direction, it is desirable to consider the inclination of the geomagnetic sensor itself. Although a tilt sensor for measuring the inclination of the geomagnetic sensor is used, the sensitivity of the tilt sensor varies from chip to chip. Therefore, it requires much effort to check and correct sensitivity in a product line.
Patent Document 1: Japanese patent laid-open No. 2004-012416.